Abstract
The electronic structure and photochemistry of copper formate clusters, CuI 2(HCO2)3 − and CuII n(HCO2)2n+1 −, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with CuI and CuII copper ions is observed. For the CuI species, transitions between copper d and s/p orbitals are recorded. For stoichiometric CuII formate clusters, the spectra are dominated by copper d–d transitions and charge‐transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low‐lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In CuII(HCO2)3 −, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge‐transfer excitations, specific excited‐state reaction channels are observed in addition, such as formyloxyl radical loss. In CuI 2(HCO2)3 −, the system relaxes to a local minimum on an excited‐state potential‐energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a CuII(HCO2)3Cu0− biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.
Highlights
With global warming becoming an ever-increasing problem, efficient activation and transformation of carbon dioxide becomes a desirable option for carbon capture and usage (CCU)
The electronic structure of copper formate clusters has been investigated in the gas phase by using UV/Vis spectroscopy between 0.9 and 5.5 eV
For the paddlewheel structure of isomer 2, CuÀCu charge transfer initiates the photochemical loss of a neutral copper atom, which competes with statistical dissociation in the electronic ground state
Summary
Gas-phase investigations of well-defined, size-selected model systems, in combination with quantum chemical calculations, are useful.[18,19,20] gas-phase approaches can be translated through size-selected cluster deposition to study surface effects within heterogeneous catalysis.[21,22] In the gas phase, it was shown that the activation of methane to methanol might take place on a CuO+ center.[23]. The electronic structure of small, neutral copper clusters was investigated in a neon matrix by using spectroscopy techniques within the UV/Vis range.[43,44,45] In well-defined gas-phase experiments, the electronic structure of a ligated CuO+ core thought to be responsible for many reactions in nature within coppercontaining enzymes has been characterized,[46] along with detailed calculations on the bare CuO+ ion.[47] In the closely related Cu(NO3)3À anion, ligand-to-metal charge-transfer (LMCT) transitions and transitions within the nitrate ligand were observed in the UV region.[48] Excitation causes the evaporation of a neutral NO3 radical, similar to collision-induced dissociation (CID) experiments, which indicates efficient internal conversion to the electronic ground state.[48,49,50]. To compute electronic transitions in larger clusters, time-dependent (TD) DFT with the BMK functional was applied, which was chosen based on its performance compared with EOM-CCSD for copper formate in oxidation state + II, see Table S5 in the Supporting Information.
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